1. Field of the Invention
The invention relates to a high-voltage insulating system for electric machines, comprising a turn-to-turn insulation and a main insulation, in which turn-to-turn insulation an insulating tape, exhibiting at least three layers, is wound, onto a preferably enamel-insulated conductor element, around the conductor element and is bonded to the latter, which insulating tape comprises a center layer which essentially contains mica and which is covered on both sides by plastic foils, the conductor elements, combined into windings or rods, are surrounded by a main insulation and installed in the stator of the electric machine and are impregnated as a whole with impregnating resin which is subsequently cured in heat.
In this connection, the invention relates to a prior art obtained, for example, from Swiss Patent Specification 559 451.
2. Discussion of Background
For insulating the stator windings of rotating machines, particularly high-voltage motors, the post-impregnation technique has become highly successful in recent years. The winding elements for these machines are constructed as so-called integral coils for structural reasons. The winding insulation is extremely stressed, particularly in the case of abrupt voltage loading (switching and other overvoltages with short front duration), and represents the weakest part of the stator winding with respect to insulation. In the past, numerous faults which are attributable to a failure of the turn-to-turn insulation have been described in the literature. The insulation between adjacent electric turns (turn-toturn insulation) must meet several requirements in order to be able to ensure high operational reliability.
The requirements are set with respect to the various loads to which the turn-to-turn insulation is already abruptly subjected during the processing of the insulating material which is in most cases tape-shaped (winding the tape around the winding wire, section wire), shaping the coils (that is to say bending on edge and on the flat) and during the further production phases (for example impregnation, curing), various tests and then continuously or repetitively in operation.
In consequence, a tape used for insulating the turns must exhibit high flexibility and, at the same time, high mechanical load carrying capability. Previous solutions were lacking in this respect since these in some cases contradictory requirements could only be partially met. The good characteristics of the mica layer such as, for example, its electrical barrier effect, was in some cases distinctly reduced due to mechanical stresses.
Furthermore, the turn-to-turn insulation must exhibit high corona resistance at increased (operating) temperature. According to information in the literature, the dielectric long-term behavior of previous solutions has been found to be inadequate in many cases.
A sufficiently high surge withstand capability (also of the aged turn-to-turn insulation) is very essential. The latest IEC recommendations prescribe a minimum surge withstand capability of 0.5 (4 U.sub.N +1) kV (crest value) , where U.sub.N =rated voltage in kV.sub.rms, per coil. For a machine with U.sub.N =15 kV, this means a turn-to-turn insulation having a surge withstand capability of more than 30.5 kV.
In addition, the components of the turn-to-turn insulation must be chemically resistant to the impregnating resins/enamels. Furthermore, good bonding of the turn-to-turn insulation both to the (enameled) winding wire and to the so-called main insulation of the coils (that is to say insulation with respect to earthed iron) is required in order to achieve an overall homogeneous structure of the main/turn-to-turn insulation.
It is also of economic significance that the turn-to-turn insulation needs space of as small a volume as possible (in comparison with the "active material" such as copper wire) and it can be inexpensively produced and applied.
The turn-to-turn insulation according to Swiss Patent Specification 559 451 consists of four layers: a mica layer of about 100 .mu.m, a thin glass fabric (as support for the mica layer) and cover layers on both sides, having a thickness of about 8-10 .mu.m of polycarbonate foil. The center layer of the insulating tape, which consists of glass fabric and mica, is bonded to the two cover foils by means of a solventless bonding agent, preferably epoxy resin in the B state. After the conductor element insulation has been applied and the insulated conductors have been formed into coils, excess resin emerges at the points of overlap of the tape during the pressure and heat treatment and bonds the insulating tape to the single conductors and the single conductors to one another. After the curing of the resin, the conductor bundle, which is pre-strengthened in this manner, is provided with the (dry) main insulation and this is then further treated in the so-called post-impregnation process.
This known method has been most successful in the past but required a comparatively time-consuming intermediate step which could not be omitted. Without it, there is no guarantee that the resin also emerges to the desired and necessary extent from the said mica/glass fabric layer at the points of overlap of the insulating tape. In addition, the resin system of the main insulation had to be accurately matched to the cover foils in order to ensure perfect bonding without cavities between the conductor element insulation and main insulation.
For this reason, later proposals for insulating tapes for turn-to-turn insulation, for example in accordance with the conference report "Teilleiterisolation auf Glimmerbasis for Hochspannungsmaschinen" (mica-based conductor element insulation for high-voltage machines), distributed at the 3rd International ASTA Symposium, May 5-7, 1987 in Baden near Vienna, have omitted one of the two cover foils, extremely thin cover foils being used in the case of multi-layer insulating tapes.
However, the bonding problem demonstrated above is only partially solved by means of these known insulating tapes.